Process for very rapid vegetable tanning of sole leather



3,086,834 PROCESS FOR VERY RAPE VEGETABLE TANNW G F SGLE LEATHER Qarlo Bellucci-Sessa, Milan, Italy, assignor to Ledoga S.p.A., Milan, Italy, a corporation of Italy N0 Drawing. Filed Nov. 3, 1960, Ser. No. 66,938 Claims priority, application Italy Nov. 26, 1959 9 Claims. (Ci. 8-?432) Sole leather is generally obtained by dipping heavy hides, which have previously been depilated by means of treatment with slaked lime solutions alone or mixed with sodium sulfide and then completely or partially delimed with organic or inorganic acids or salts, into solutions of tanning extracts of vegetable nature.

As a rule, pyrogallic or pyrocatechinic tanning extracts are used either alone or in mixture; at first tanning is carried out with very dilute and aged solutions and then continued with gradually more concentrated and fresher mixtures.

The reason for this procedure is mainly due to the fact that the vegetable tanning materials penetrate very slowly into hides, owing to the astringent nature of the extracts. If used in high concentrations, the latter would rapidly fix to the surface of the hide, thus forming an obstacle to their further penetration and producing a rough and easily cracking grain. This fault reduces the value of leather to an extent that it may even be unsaleable.

To avoid this result large quantities of liquid must be used, thus necessitating the use of very large containers. Furthermore, the tanning liquors must be continuously refilled with consequent heavy waste of work and loss of large amounts of tanning substances due to fermentation and precipitation.

Under such working conditions, tanning is slow and takes from one to two months for the faster tanning processes and much longer for the slow ones. This also means holding up large amounts of capital in stocks of hides and tanning materials.

Many attempts have been made to speed up tanning by reducing the astringency of the tanning liquors. This depends on many factors, often difiicult to control, such as the nature of the tanning extracts, temperature, acidity and salt content, viscosity and density of said liquors. For various reasons the results of such attempts have never been completely satisfactory.

Besides changes in the tanning liquors, investigators have turned their attention to the structure of hides, moditying it in the attempt to make penetration more easy. The methods followed to achieve this purpose were generally linked to pre-tanniug of the grain. Among the methods used, mention must be made of pre-tanning with synthetic or similar tannins, chromium salts, aluminum salts, aldehydes, etc. Here also, the results were incomplete, especially as, although the tanning time was markedly reduced, though not to the desired degree, the resultting leather did not possess the full properties of a pure vegetable tanned leather. Furthermore, this affected the yield of the finished product, which is an important factor from the economic point of View.

The object of the present invention is to decrease the time of preparation and tanning oi hides to a maximum of 3 to 4 days when compared with 36 to 48 days commonly required by known rapid tanning methods.

Fatent 2 The procedures of rapid vegetable tanning now in use generally consists of the following stages:

Stage 1, soaking or washing of the By my new process the time for stages 3, 4, 5, 6 and 9 is modified or eliminated as follows, whereas the times for stages 1, 2, 7 and 8 remain unchanged:

(1) Stage No. 3 (deliming) can be eliminated, as the process is carried out on non-delirned hides.

(2) Stages 4, 5 and 6, which require from 24 to 36 days in the usual processes, are replaced by a single 2- day stage.

(3) Stage No. 9 (finishing and drying) can be shortened by at least 3 days.

Therefore, the entire tanning process can be reduced to only 11 days, thus saving from 25 to 37 days.

Under these conditions, the disadvantages connected with the conventional rapid tanning process, such as loss of tanning mate-rial, heavy work, investment of capital for raw materials and hides, are overcome, or at least considerably reduced.

The process claimed by the present invention is based on inactivation of the protein amine groups in the pelts. According to the liaterature, when hides are treated with nitrous acid (that is, with an alkaline nitrite in the presence of an even weak acid), their capacity of fixing tanning substances is reduced. The amine groups of the end chains and, in particular, those of lysine, are diazotized and transformed under loss of nitrogen, into groups no longer reacting with the tanning materials. If this operation, known as collagen deamination, is limited to the external portions of the pelts, it is obvious that the fixing capacity of tannin is reduced, thus avoiding the harmful occurrence of dead tanning which occurs even in concentrated solutions. This is shown by the inability of tanning substances to diffuse through the whole cross-section of the hide. Nitrous acid is also formed by direct reaction between sodium nitrite and chestnut extract, as the latter is Well known to possess a high natural organic acidity. Sodium nitrite is present through the whole crosssection of the hide, but the deamination reaction can occur only at the outer surface of the latter, having acidic reac tion, while the reaction cannot take place in the interior, because of the high alkalinity due to lime. As previously stated, deamination of the grain afiects the fibres. The grain is therefore previously fixed with small amounts of aldehydes having tanning properties, such as formaldehyde, glutaraldehyde, dialdehyde starch etc. in order to avoid any loss of elasticity of the grain.

Furthermore, penetration of the tanning substances into the hide is favoured by the alkalinity of the internal portion of the latter, as this reduces the astringency of the tanning substances. It is therefore also possible to obtain the advantage of not reducing the irreversible fixation of tannin which, at a pH of 4.5 to 5, i.e. under the conditions present during practically the whole tanning process, is not very different from that occurring when non-deaminated collagen is used. The process claimed consists of three stages:

(1) Imbibition. (2) Surface deamination of the skin. (3) True and proper tanning.

Inbibitin.This operation is carried out on the fleshed, non-delimed pelts, whereby the liming and previous operations are performed as usual. The non-delimed pelts are Washed in running water in rotating drums for a short time, in order to eliminate any lumps of lime and residual fibres from the fleshing. Water is then added in amount sufficient to cover the pelts; a small quantity of sodium nitrite is dissolved in the water and the drum closed until the nitrite hs penetrated through the whole cross-section of the pelts. This is ascertained by cutting a section of pelt crosswise in the thickest part, wetting the internal layer in dilute hydrochloric or sulfuric acid solution, and checking the result with starch-iodide paper (blue colour).

This result is obtained after a few hours of rotation, the duration of which varies depending upon the speed of the drum, the thickness of the pelts and the concentration of nitrite. Nevertheless, at the end of drummining it is advisable to leave the pelts for some hours in the same bath, after which the residual solution is removed.

At the end of this stage the pelt appears very firm with a cross-section of t-ranslucid aspect.

In order to obtain a satisfactory leather, particularly as regards colour, it is advisable, before imbibition, to wash the pelts in running water for 20-30 minutes and to carryout partial deliming of the outer surface of the pelts. Both these operations are performed to ensure that the external surfaces of the pelts are free from lime residues and thoroughly clean.

Surface deamination of the pelts.-Water is added to the same drum in which the imbibition process was performed, and rotation started. A calculated amount of mineral or organic acid, previously diluted with water, is added through the hollow axle. This amount is such as to:

(1) Neutralize the alkalinity present in the external surfaces of the pelts;

(2) Permit freeing of the nitrous acid from the nitrite absorbed in the external surfaces of the pelts.

In order to avoid that decomposition of the nitrite and neutralization of the lime may affect the inner layers, the amount of acid used is usually very small, although varying in relation to the strength of the acid used.

The pelts are rotated for several hours and at the end of this period the pH of the solution is tested; it should be below neutrality, while the internal section of the pelt should be distinctly alkaline.

v The deamination leads to an attack on the fibres by the nitrous acid and the former tend to lose their elasticity which is an essential property of the grain. This untoward effect can be avoided by performing, before surface deamination of the hide, a grain-fixing stage by means of an aldehyde, utilizing the alkalinity of the pelt treated as described above.

Deamination of the grain can be obtained after its fixation with aldehydes, merely by utilizing the natural organic acidity of the vegetable extract, particularly chestnut, which is itself suificient for releasing the nitrous acid. The chestnut extract is added according to the method described below :for the tanning stage.

Tanning.At this point the liquid is discharged and the pelts are washed in running water for minutes, after which fresh water is added to the pelts in quantity sufficient to ensure that the successive addition of tanning extract produces a solution of the required density.

weight on the weight of the pelts, varies from 40% to 70%, according to the degree of nervousness required from the leather. The pH of the tanning solution should not be less than 4.6.

The extract in contact with the pelts rapidly passes through the grain, causing only slight wrinkling which gradually disappears as the tanning proceeds.

Penetration is complete after about 7 to 8 hours, but the time of rotation should be prolonged in order to completely fix the tanning substance on the pelts and to fill in the spaces between the fibres, that is, until the density of the residual liquor remains constant.

The density of the residual liquor falls markedly at the end of the tanning process, while the pH remains about the same as at the beginning. The residual liquor, still possessing tanning properties, although to a reduced extent, may be reused, if desired, for processing other articles. By operating under the above conditions, the quantity of tannin-g substance absorbed by the pelt is not a function of the exhaustion of the residual liquor but rather of the amount of extract which was originally present. After the tanning process the pelts are pressed, retanned and finished as usual, except that the finishing and drying can be limited to no more than 4 days, instead of the usual 7 days.

The physical and analytical properties of the leather obtained with the extra-rapid tanning process here described do not difier in any way from those of equal type leather obtained with the usual tanning processes. It is understood that possible obvious variants of the above-mentioned process remain within the scope of the present invention.

Example 1 kg. of heavy non-delimed pelts are placed in a tanning drum and rotated with litres of water, in which 2 kg. of crystalline sodium nitrate are dissolved.

The drum is allowed to rotate for 10 minutes and the pH is then checked; it should be 6.5. Rotation is continued for 5 hours and at the end the pH is again checked and should be 10.5. The residual solution is discarded.

150 litres of water are added to the pelts in the same tanning drum, along with 0.130 kg. of sodium hydroxide solution (38 B.) and 0.600 kg. of 98% acetic acid, followed 30 minutes later by a further 0.600 kg. of 98% acetic acid. The pH of the solution is checked and should be 4.6. The drum is rotated for further 30 minutes, after which the pelts are left standing for 12 hours. The solution is removed and 154 kg. of chestnut extract (19 B6.) are added at pH 4.6, corresponding to 56 kg. of commercial powdered chestnut extract.

' The drum is then rotated for 48 hours, that is, until the liquid has reached a constant density of 8 B. and pH 5.

The solution is removed and the pelts washed for 5 minutes; they are then taken from the drum, pressed, retanned, finished and dried as usual.

Example 2 The imbibition is carried out as in Example 1, except that the amount of sodium nitrite is reduced to 1.5 kg. After removing the solution, 150 kg. of water and 0.700 kg. of crystalline oxalic acid are added; the drum is then rotated for 2 hours and the pH of the solution checked (4.5-4.6). The-solution is then discharged.

Tanning in the drum is then performed as for Example 1, using 163 kg. of tanning extract at 16 B., pH 4.6 (equivalent to 50 kg. of powdered extract). After. rotating for 48 hours the pelts are completely tanned; they are then removed, pressed and finished with the usual methods.

Example 3 The imbibition and deamination stages are identical with those of Example 2. The amount of tanning extract used is 165 kg. at 19 Be, pH 4.6, equivalent to 60% of powdered tanning extract.

At the end of 48 hours rotation in the cask, the pelts are treated as usual.

Example 4 100 kg. of heavy non-delimed pelts are rotated in the tanning drum with 200 liters of water in which 2 kg. of crystalline sodium nitrite are dissolved.

After 10 minutes rotation, the pH is 6.5 and the rotation is continued for a further hours. The drum is then stopped for 12 hours. Rotation is again started and 0.700 kg. of commercial hydrochloric acid (36-37%), previously diluted in 1.400 kg. of water, are rapidly added to the same bath through the hollow axle. The resulting pH of the solution is 3.6-3.8. Rotation is continued a further 3 hours, that is, until the pH of the solution rises to 4.8-5, after which the liquid is removed. 1.67 kg. of tanning extract solution at 22 B., pH 4.6, equivalent to 70% of powdered tanning extract are added to the same drum. The latter is rotated for 40 hours, after which the completely tanned hides are finished as usual.

Example 5 100 kg. of non-delimed scraped oif pelts, are thoroughly washed in cold running water for 20-30 minutes in a rotating tanning drum. Water suflicient to cover the pelts is added in the same drum, along with a small quantity of commercial delimer, and the drum rotated until the external surfaces are partly delimed. This liquid is run off and the pelts then treated with 200 kg. of water in which 2.5 kg. of sodium nitrite have been dissolved. The drum is then rotated for the time necessary for the nitrite to penetrate through the pelts. After adjusting the pH to approximate neutrality, about 1% of formaldehyde diluted 1:10 is added dropwise, rotating the pelts for about 2 /2 hours.

Vegetable tanning is carried out at this point as described in the previous examples.

What I claim is:

1. A process for the rapid vegetable tanning of leather which comprises imbibing previously limed pelt in the non-delimed condition with a dilute aqueous solution containing an alkali metal nitrite in a Weight ratio of 2-3% on the weight of pelt, adding to the so treated pelt an aqueous solution containing an acid selected from the class of organic and mineral acids to pH 4.5 to 6.5 and tanning the pelt in a concentrated aqueous solution of a vegetable tanning agent.

2. A process as in claim 1, wherein tanning is carried out in a single tanning liquor of density between 16 and 22 B. and at a pH higher than 4.6 and using an amount of dry vegetable tanning extract corresponding to 40 to by weight of the pelt.

3. A process as in claim 1, wherein the vegetable tanning extract is chestnut extract.

4. A process for the rapid vegetable tanning of leather which comprises imbibing previously limed pelt in the non-delimed condition with a dilute aqueous solution containing sodium nitrite in a weight ratio of 2-3% on the Weight of pelt, adding to the so treated pelt an aqueous solution of acetic acid to pH 6.5, and thereafter tanning the pelt in a concentrated aqueous solution of chestnut extract.

5. A process for the rapid vegetable tanning of leather which comprises imbibing previously limed pelt in the non-delimed condition with a dilute aqueous solution containing an alkali metal nitrite in a weight ratio of 23% on the weight of pelt, and tanning the pelt with an acidic chestnut extract.

6. A process for the rapid vegetable tanning of leather which comprises imbibing previously limed pelt in the non-delimed condition with a dilute aqueous solution of an alkali metal nitrite until the nitrite has penetrated the pelt, and tanning the pelt under acid conditions with a concentrated solution of a vegetable tanning agent, whereby protein amine groups in the surface fiber layers are inactivated by the liberated nitrous acid and penetration by the tanning agent is facilitated.

7. Process according to claim 6, wherein the limed pelt is first partially delimed at the surfaces of the pelt.

8. Process according to claim 6, wherein, following the nitrite treatment, the pelt is subjected to the action of an aldehyde to fix the surface grain of the pelt.

9. Process according to claim 6, wherein, following the nitrite treatment, the pelt is subjected to the action of an aldehyde to fix the surface grain of the pelt, and wherein the tanning is carried out with chestnut extract.

References Cited in the file of this patent UNITED STATES PATENTS Moore et a1 Aug. 27, 1957 OTHER REFERENCES 

1. A PROCESS FOR THE RAPID VEGATABLE TAINING OF LEATHER WHICH COMPRISES IMBIBING PREVIOUSLY LIMED PELT IN THE NON-DELIMED CONDITION WITH A DILUTE AQUEOUS SOLUTION CONTAINING AN ALKALI METAL NITRITE IN A WEIGHT RATIO OF 2-3% ON THE WEIGHT OF PELT, ADDING TO THE SO TREATED PELT AN AQUEOUS SOLUTION CONTAINING AN ACID SELECTED FROM THE CLASS OF ORGANIC AND MINERAL ACIDS TO PH 4.5 TO 6.5 AND TAINING THE PELT IN A CONNECTRATED AQUEOUS SOLUTION OF A VEGATABLE TAINING AGENT. 